552 Combined loss of integrins α1β1 and α2β1 results in reduced angiogenesis in mice

Loss of integrin α1β1 in mice results in reduced tumor size and tumor angiogenesis, whereas loss of integrin α2β1 leads to increased angiogenesis in wounds and tumors. These findings show that integrin α1β1 and α2β1 have opposing effects on angiogenesis. However, the synergistic outcome of the opposing effects of these two integrins in angiogenesis remains elusive. We analyzed wound and tumor vascularization in mice with constitutive ablation of integrin subunits α1 and α2, thereby removing the α1β1 and α2β1 receptors (dKO). Compared to controls, dKO mice displayed reduced wound angiogenesis at day 3 and 7 post wounding. These results were confirmed by reduced angiogenesis of sponges implanted into the thoracic cavity of dKO mice compared to controls. Similarly, intradermal injection of B16 F1 melanoma cells induced a reduction in tumor angiogenesis in dKO mice compared to controls. This reduction was a result of increased levels and activity of matrix metalloproteinase-9 (MMP-9), that cleaves circulating plasminogen to release increased amounts of angiostatin, a potent inhibitor of endothelial cell proliferation. Concomitantly, plasma from tumor bearing dKO mice displaying increased levels of angiostatin reduced HUVEC proliferation. Of note, endothelial cells in dKO wounds and tumors showed reduced proliferation. These results were confirmed in an ex vivo aortic ring angiogenesis model, where significantly fewer and thinner sprouts emerged from dKO aortic explants, and they exhibited reduced endothelial cell proliferation and fewer branch points. These observations suggested defects in both primary and secondary sprouting in the dKO explants as compared to controls. In summary, our observations show that the combined loss of integrins α1β1 and α2β1 in mice results in a reduction of wound and tumor angiogenesis due to enhanced MMP-9 activity and circulating angiostatin leading to reduced endothelial cell proliferation similar to the mechanism observed in the α1-deficient mice.